Production of lambs of predetermined sex after the insemination of ewes with low numbers of frozen - thawed sorted X- or Y-chromosome-bearing spermatozoa
The fertilizing ability of sex-sorted frozen–thawed ram spermatozoa was assessed after insemination of mature Merino ewes at a synchronized oestrus. Ewes were inseminated into the uterus or utero–tubal junction (UTJ) with a total of 140 × 106 unsorted (control) or 2–4 × 106 sorted (X or Y) frozen–thawed ram spermatozoa 54�to 57 hours after removal of progestagen-impregnated pessaries and an injection of 400 IU of pregnant mare serum gonadotrophin (Folligon®, Intervet). The spermatozoa were separated into X- and Y-chromosome-bearing spermatozoa after analysis with a modified high-speed cell sorter (SX MoFlo®). The number of ewes pregnant after insemination with unsorted frozen–thawed spermatozoa was significantly higher (26/48; 54.3%) than for ewes inseminated with either X- (12/48; 25.0%) or Y-sorted spermatozoa (7/48; 14.6%) (P